Shrimp and prawn cultivation and trade is a very important activity all over the world. The main species under cultivation are Penaeus monodon (Giant tiger prawn, Jumbo tiger prawn, Jumbo tiger shrimp, Black tiger prawn, Blue tiger prawn, and Grass shrimp, etc.), mainly cultivated in Asia, with an aquaculture production of about 600,000 tons in 2003; and Penaeus vannamei (Whiteleg shrimp, white shrimp), mainly cultivated in the Americas and in China and Thailand, with an aquaculture production that is comparable to P. monodon. For those species, aquaculture is far more important than capture.
Increasing demands for aquaculture production mean increasing pressure for the development of more efficient production systems. More and more, modern genetics are used to support stock improvement and breeding programs (Hulata, 2001). Genomic research and gene mapping developed fast during recent times. DNA markers have been characterized for use in establishing pedigrees, linkage mapping and identifying Quantitative Trait Loci (QTLs).
As most Penaeus sp. are sexually dimorphic (Hansford and Hewitt, 1994), a lot of effort has been made to find a reliable sex marker, which could help in setting up and maintaining monosex cultures. Several groups developed linkage maps, mainly based on the use of AFLP markers (Moore et al., 1999; Wilson et al., 2002). Pérez et al. (2004) published a sex-specific linkage map of the white shrimp Penaeus vannamei. However, they did not identify a sex-linked marker or linkage group. Li et al. (2003) disclosed a sex-specific linkage map of Penaeus japonicus, with a presumed sex marker on the maternal linkage map. Zhang et al. (2006) published a linkage map of P. vannamei, with sex-linked microsatellite markers present on the female map. In the latter two cases, however, the sex-marker association was not challenged among genetically unrelated individuals.
It should be stressed that the relatively low number of sample meioses within designed populations (e.g., half-sib families) leads to relatively long stretches of chromosomes being in Linkage Disequilibrium (LD). Consequently, in such linkage studies, the observed high LD between marker and sex dimorphism results from the nature of the population rather than from the tight physical linkage. Therefore, markers found via linkage analysis to be in LD with the sex often fail to discriminate between the two sexes in a population of unrelated individuals. Indeed, Li et al. (2003) admit that the presumed marker is not necessarily linked to a sex-specific sequence, and no sequence data are disclosed. Moreover, Zhang et al. (2004) were unable to identify sex-specific markers in Penaeus chiniensis using the AFLP approach. Likewise, Khamnamtong et al. (2006) could not identify sex-specific markers in Penaeus monodon. 